The pumps are handy for boat building, not so much for laminating guitar parts. As Barry mentioned their accuracy fades over time unless you keep them maintained. For the small quantities of epoxy needed to laminate back and side sets I use an old triple beam balance and zero out the container I'm adding the epoxy into. I first measure the hardener into the cup and then add the resin. that makes it easier to compensate for any mismeasurement of the hardener. For very small quantities I have filled dropper bottles with hardener and resin and then count the drops to get the right mix.
Epoxy goes a surprisingly long way, so if I am not sure how much I will need I will err on the side of too much, but try to have another little side project to use the extra on.
For boat building and other high use activities, I think most people do just leave the pumps in place and then clean them up when changing cans. I initially used them but with my slow rate of use they "failed" before the first gallon of resin was used up.
Some boatbuilders who use larger quantities will also forgo pumps and instead mark two containers with "fill lines" and then dump into a third container to mix in.
One suggestion I would make is to buy a box of disposable nitrile gloves and always wear them when working with epoxy. That stuff has a way of getting on your hands and cleaning it off with alcohol as I have done is not a good idea. I also like to cover my bench with paper when working with the stuff and wear old glue stained clothes that I don't mind getting ruined.
But it is good stuff!
